1. Field of the Invention
The invention relates to a controllable vibration damper for motor vehicles. More specifically, the present invention relates to a controllable vibration damper having a cylinder containing a damping fluid, a piston rod axially displaceably arranged in the cylinder in a sealed manner, and a damping piston fastened to the piston rod and dividing the cylinder into two working spaces. The vibration damper also has a damping valve provided with a valve seat which controls the effective cross section of a damping passage of the main stage via an axially displaceable valve body.
2. Description of the Related Art
Prior art vibration dampers, such as, for example, those disclosed in German reference DE-A 38 00 865, include a piston in which the entire quantity of damping fluid exchanged between two working spaces flows via a restricting cross section, the size of which is determined by an electromagnetic setting device primarily comprising a solenoid and an armature. The piston is in this case subjected to flow from two sides, without nonreturn valves being provided. This prior art device is a directly controlled piston valve in the piston of a vibration damper. A problem with this prior art design is that the spring forces and magnetic forces required to overcome the flow forces of the damping fluid and to stabilize the system have to be very high.
Other prior art vibration dampers are also known, for example, from reference WO/85/04698, in which a precontrolled damping valve in the piston of the vibration damper is provided with four nonreturn valves. In these prior art vibration dampers, flow is directed through the precontrolled damping valve in only one direction. That is to say, the nonreturn valves must make the entire oil flow unidirectional. Establishing a unidirectional flow of the oil relates in this case to the main stage. A problem with these prior art designs is that either large nonreturn valves are required, which cannot be structurally accommodated very well in the vibration damper, or the extent to which the pressure can be lowered when there are high volumetric flows through the precontrolled damping valve is restricted.
Yet another type of prior art vibration dampers are known, for example, from references DE-A 44 18 972 and EP-A 0,364,757, in which a valve body of a damping valve subjected to flow from alternate sides in the vibration damper is controlled by a precontrolling stage. The damping valve is in this case subjected to flow from alternate sides in the main stage, while the valve body is subjected to unidirectional flow in the compression and rebound stages to establish a unidirectional flow of the precontrolling stage. This prior art structure requires four nonreturn valves, which establish the corresponding unidirectional flow in the precontrolling stage. A problem with this prior art device is that, due to the number of corresponding nonreturn valves, a considerable constructional expenditure is required and the response and timing of the control of the damping valve of the vibration damper are compromised.
Yet another prior art vibration damper disclosed in reference DE 37 19 113 C1 which includes a controllable shock absorber which has an infinitely adjustable damping valve. To reduce the controlling energy, the damping valve is provided with a precontrolling means in which the valve body used has a restricting bore extending from an end face to which pressure is applied to a rear side which is controlled by an axially constantly displaceable control needle.